This invention is in the field of plant molecular biology. More specifically, this invention pertains to nucleic acid fragments encoding enzymes involved in degradation of branched-chain amino acids in plants and seeds.
Amino acids, in addition to their role as protein monomeric units, are energy metabolites and precursors of many biologically important nitrogen-containing compounds, notably heme, physiologically active amines, glutathione, nucleotides, and nucleotide coenzymes. Excess dietary amino acids are neither stored for future use nor excreted. Rather they are converted to common metabolic intermediates such as pyruvate, oxaloacetate, and alpha-ketoglutarate. Consequently, amino acids are also precursors of glucose, fatty acids, and ketone bodies and are therefore metabolic fuels.
Hydroxymethylglutaryl-CoA lyase (EC 4.1.3.4), also called HMG-CoA lyase, is involved in the degradation of leucine, and participates in butanoate metabolism, and in the synthesis and degradation of ketone bodies. HMG-CoA lyase catalyzes the final step of ketogenesis and leucine catabolism in the mitochondrial matrix. The first reported HMG-CoA lyase gene was from Pseudomonas mevalonii (Anderson, D. H. and Rodwell, V. W. (1989) J Bacteriol. 171:6468-6472). The active site of the Pseudomonas mevalonii HMG-CoA lyase has been identified. Cys-237 is required for catalysis (Hruz, P. W. et al. (1992) Biochemistry 31:6842-6847). To date, HMG-CoA lyase has not been described in plants.
3-Hydroxyisobutyrate dehydrogenase (EC 1.1.1.31) catalyzes the NAD-dependent, reversible oxidation of 3-hydroxbutyrate to methylmalonate (Rougraff, P. M. et al. (1989) J. Biol. Chem. 264:5899-5903). In animals, it is a homodimeric mitochondrial protein involved in valine catabolism. In Pseudomonas aeruginosa (encoded by the mmsB), it is involved in the-distal valine metabolic pathway (Steele, M. I. et al. (1992) J. Biol. Chem. 267:13585-13592). The sequence of 3-hydroxyisobutyrate dehydrogenase from eukaryotic and prokaryotic sources show that this enzyme has been well conserved throughout evolution. The pathway of valine catabolism ultimately leads to the production of succinyl-SCoA. Succinyl-SCoA can be converted to pyruvate via the TCA cycle and then to glucose. Thus, this enzyme is needed, along with several others in the catabolic pathway, to interconvert the carbon skeleton of valine into other useful metabolites. 3-hydroxyiso-butyrate dehydrogenase has not been isolated from plants yet, although rice ESTs encoding portions of this gene are present in the GenBank database.
Involved in the processing of leucine, isovalyryl-CoA dehydrogenase (EC 1.3.99.10) uses FAD to convert isovalyryl-CoA to beta-methylcrotonyl-CoA. This enzyme is found in the mitochondria and has similarity with other acyl-CoA dehydrogenases (long chain acyl-CoA (LCAD), short chain acyl-CoA (SCAD), and medium-chain (MCAD) acyl-CoA dehydrogenases). The structural relatedness of these enzymes suggests that they are members of a gene family that shares a common ancestral gene (Matsubara et. al. (1989) J Biol Chem 264(27):16321-16331). Rice and oat ESTs exist in the GenBank database but the enzyme has not yet been isolated from plants.
The instant invention relates to isolated nucleic acid fragments encoding enzymes involved in degradation of branched-chain amino acids. Specifically, this invention concerns an isolated nucleic acid fragment encoding a hydroxymethylglutaryl CoA oxidase, a 3-hydroxyisobutyrate dehydrogenase or an isovalyryl-CoA dehydrogenase and an isolated nucleic acid fragment that is substantially similar to an isolated nucleic acid fragment encoding a hydroxymethylglutaryl CoA oxidase, a 3-hydroxyisobutyrate dehydrogenase or an isovalyryl-CoA dehydrogenase. In addition, this invention relates to a nucleic acid fragment that is complementary to the nucleic acid fragment encoding hydroxymethyl-glutaryl CoA oxidase, 3-hydroxyisobutyrate dehydrogenase or isovalyryl-CoA dehydrogenase.
An additional embodiment of the instant invention pertains to a polypeptide encoding all or a substantial portion of a branched-chain amino acid degradation enzyme selected from the group consisting of hydroxymethylglutaryl CoA oxidase, 3-hydroxyisobutyrate dehydrogenase or isovalyryl-CoA dehydrogenase.
In another embodiment, the instant invention relates to a chimeric gene encoding a hydroxymethylglutaryl CoA oxidase, a 3-hydroxyisobutyrate dehydrogenase, or an isovalyryl-CoA dehydrogenase, or to a chimeric gene that comprises a nucleic acid fragment that is complementary to a nucleic acid fragment encoding a hydroxymethylglutaryl CoA oxidase, a 3-hydroxyisobutyrate dehydrogenase, or an isovalyryl-CoA dehydrogenase, operably linked to suitable regulatory sequences, wherein expression of the chimeric gene results in production of levels of the encoded protein in a transformed host cell that is altered (i.e., increased or decreased) from the level produced in an untransformed host cell.
In a further embodiment, the instant invention concerns a transformed host cell comprising in its genome a chimeric gene encoding a hydroxymethylglutaryl CoA oxidase, a 3-hydroxyisobutyrate dehydrogenase, or an isovalyryl-CoA dehydrogenase, operably linked to suitable regulatory sequences. Expression of the chimeric gene results in production of altered levels of the encoded protein in the transformed host cell. The transformed host cell can be of eukaryotic or prokaryotic origin, and include cells derived from higher plants and microorganisms. The invention also includes transformed plants that arise from transformed host cells of higher plants, and seeds derived from such transformed plants.
An additional embodiment of the instant invention concerns a method of altering the level of expression of a hydroxymethylglutaryl CoA oxidase, a 3-hydroxyisobutyrate dehydrogenase, or an isovalyryl-CoA dehydrogenase in a transformed host cell comprising: a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding a hydroxymethylglutaryl CoA oxidase, a 3-hydroxyisobutyrate dehydrogenase, or an isovalyryl-CoA dehydrogenase; and b) growing the transformed host cell under conditions that are suitable for expression of the chimeric gene wherein expression of the chimeric gene results in production of altered levels of hydroxymethylglutaryl CoA oxidase, 3-hydroxyisobutyrate dehydrogenase or isovalyryl-CoA dehydrogenase in the transformed host cell.
An addition embodiment of the instant invention concerns a method for obtaining a nucleic acid fragment encoding all or a substantial portion of an amino acid sequence encoding a hydroxymethylglutaryl CoA oxidase, a 3-hydroxyisobutyrate dehydrogenase, or an isovalyryl-CoA dehydrogenase.
A further embodiment of the instant invention is a method for evaluating at least one compound for its ability to inhibit the activity of a hydroxymethylglutaryl CoA oxidase, a 3-hydroxyisobutyrate dehydrogenase, or an isovalyryl-CoA dehydrogenase, the method comprising the steps of: (a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding a hydroxymethylglutaryl CoA oxidase, a 3-hydroxyiso-butyrate dehydrogenase, or an isovalyryl-CoA dehydrogenase, operably linked to suitable regulatory sequences; (b) growing the transformed host cell under conditions that are suitable for expression of the chimeric gene wherein expression of the chimeric gene results in production of hydroxymethylglutaryl CoA oxidase, 3-hydroxyisobutyrate dehydrogenase or isovalyryl-CoA dehydrogenase in the transformed host cell; (c) optionally purifying the hydroxymethylglutaryl CoA oxidase, the 3-hydroxyisobutyrate dehydrogenase or the isovalyryl-CoA dehydrogenase expressed by the transformed host cell; (d) treating the hydroxymethylglutaryl CoA oxidase, the 3-hydroxyisobutyrate dehydrogenase or the isovalyryl-CoA dehydrogenase with a compound to be tested; and (e) comparing the activity of the hydroxymethylglutaryl CoA oxidase, the 3-hydroxyisobutyrate dehydrogenase or the isovalyryl-CoA dehydrogenase that has been treated with a test compound to the activity of an untreated hydroxymethylglutaryl CoA oxidase, 3-hydroxyisobutyrate dehydrogenase or isovalyryl-CoA dehydrogenase, thereby selecting compounds with potential for inhibitory activity.
The invention can be more fully understood from the following detailed description and the accompanying Sequence Listing which form a part of this application.
Table 1 lists the polypeptides that are described herein, the designation of the cDNA clones that comprise the nucleic acid fragments encoding polypeptides representing all or a substantial portion of these polypeptides, and the corresponding identifier (SEQ ID NO:) as used in the attached Sequence Listing. The sequence descriptions and Sequence Listing attached hereto comply with the rules governing nucleotide and/or amino acid sequence disclosures in patent applications as set forth in 37 C.F.R. xc2xa71.821-1.825.
The Sequence Listing contains the one letter code for nucleotide sequence characters and the three letter codes for amino acids as defined in conformity with the IUPAC-IUBMB standards described in Nucleic Acids Research 13:3021-3030 (1985) and in the Biochemical Journal 219 (No. 2):345-373 (1984) which are herein incorporated by reference. The symbols and format used for nucleotide and amino acid sequence data comply with the rules set forth in 37 C.F.R. xc2xa71.822.
In the context of this disclosure, a number of terms shall be utilized. As used herein, a xe2x80x9cnucleic acid fragmentxe2x80x9d is a polymer of RNA or DNA that is single- or double-stranded, optionally containing synthetic, non-natural or altered nucleotide bases. A nucleic acid fragment in the form of a polymer of DNA may be comprised of one or more segments of cDNA, genomic DNA or synthetic DNA.
As used herein, xe2x80x9ccontigxe2x80x9d refers to a nucleotide sequence that is assembled from two or more constituent nucleotide sequences that share common or overlapping regions of sequence homology. For example, the nucleotide sequences of two or more nucleic acid fragments can be compared and aligned in order to identify common or overlapping sequences. Where common or overlapping sequences exist between two or more nucleic acid fragments, the sequences (and thus their corresponding nucleic acid fragments) can be assembled into a single contiguous nucleotide sequence.
As used herein, xe2x80x9csubstantially similarxe2x80x9d refers to nucleic acid fragments wherein changes in one or more nucleotide bases results in substitution of one or more amino acids, but do not affect the functional properties of the polypeptide encoded by the nucleotide sequence. xe2x80x9cSubstantially similarxe2x80x9d also refers to nucleic acid fragments wherein changes in one or more nucleotide bases does not affect the ability of the nucleic acid fragment to mediate alteration of gene expression by gene silencing through for example antisense or co-suppression technology. xe2x80x9cSubstantially similarxe2x80x9d also refers to modifications of the nucleic acid fragments of the instant invention such as deletion or insertion of one or more nucleotides that do not substantially affect the functional properties of the resulting transcript vis-à-vis the ability to mediate gene silencing or alteration of the functional properties of the resulting protein molecule. It is therefore understood that the invention encompasses more than the specific exemplary nucleotide or amino acid sequences and includes functional equivalents thereof.
For example, it is well known in the art that antisense suppression and co-suppression of gene expression may be accomplished using nucleic acid fragments representing less than the entire coding region of a gene, and by nucleic acid fragments that do not share 100% sequence identity with the gene to be suppressed. Moreover, alterations in a nucleic acid fragment which result in the production of a chemically equivalent amino acid at a given site, but do not effect the functional properties of the encoded polypeptide, are well known in the art. Thus, a codon for the amino acid alanine, a hydrophobic amino acid, may be substituted by a codon encoding another less hydrophobic residue, such as glycine, or a more hydrophobic residue, such as valine, leucine, or isoleucine. Similarly, changes which result in substitution of one negatively charged residue for another, such as aspartic acid for glutamic acid, or one positively charged residue for another, such as lysine for arginine, can also be expected to produce a functionally equivalent product. Nucleotide changes which result in alteration of the N-terminal and C-terminal portions of the polypeptide molecule would also not be expected to alter the activity of the polypeptide. Each of the proposed modifications is well within the routine skill in the art, as is determination of retention of biological activity of the encoded products.
Moreover, substantially similar nucleic acid fragments may also be characterized by their ability to hybridize. Estimates of such homology are provided by either DNA-DNA or DNA-RNA hybridization under conditions of stringency as is well understood by those skilled in the art (Hames and Higgins, Eds. (1985) Nucleic Acid Hybridisation, IRL Press, Oxford, U.K.). Stringency conditions can be adjusted to screen for moderately similar fragments, such as homologous sequences from distantly related organisms, to highly similar fragments, such as genes that duplicate functional enzymes from closely related organisms. Post-hybridization washes determine stringency conditions. One set of preferred conditions uses a series of washes starting with 6xc3x97SSC, 0.5% SDS at room temperature for 15 min, then repeated with 2xc3x97SSC, 0.5% SDS at 45xc2x0 C. for 30 min, and then repeated twice with 0.2xc3x97SSC, 0.5% SDS at 50xc2x0 C. for 30 min. A more preferred set of stringent conditions uses higher temperatures in which the washes are identical to those above except for the temperature of the final two 30 min washes in 0.2xc3x97SSC, 0.5% SDS was increased to 60xc2x0 C. Another preferred set of highly stringent conditions uses two final washes in 0.1xc3x97SSC, 0.1% SDS at 65xc2x0 C.
Substantially similar nucleic acid fragments of the instant invention may also be characterized by the percent identity of the amino acid sequences that they encode to the amino acid sequences disclosed herein, as determined by algorithms commonly employed by those skilled in this art. Preferred are those nucleic acid fragments whose nucleotide sequences encode amino acid sequences that are 80% identical to the amino acid sequences reported herein. More preferred nucleic acid fragments encode amino acid sequences that are 90% identical to the amino acid sequences reported herein. Most preferred are nucleic acid fragments that encode amino acid sequences that are 95% identical to the amino acid sequences reported herein. Sequence alignments and percent identity calculations were performed using the Megalign program of the LASERGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Multiple alignment of the sequences was performed using the Clustal method of alignment (Higgins and Sharp (1989) CABIOS. 5:151-153) with the default parameters (GAP PENALTY=10, GAP LENGTH PENALTY=10). Default parameters for pairwise alignments using the Clustal method were KTUPLE 1, GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED=5.
A xe2x80x9csubstantial portionxe2x80x9d of an amino acid or nucleotide sequence comprises an amino acid or a nucleotide sequence that is sufficient to afford putative identification of the protein or gene that the amino acid or nucleotide sequence comprises. Amino acid and nucleotide sequences can be evaluated either manually by one skilled in the art, or by using computer-based sequence comparison and identification tools that employ algorithms such as BLAST (Basic Local Alignment Search Tool; Altschul et al. (1993) J. Mol. Biol. 215:403-410; see also www.ncbi.nlm.nih.gov/BLAST/). In general, a sequence of ten or more contiguous amino acids or thirty or more contiguous nucleotides is necessary in order to putatively identify a polypeptide or nucleic acid sequence as homologous to a known protein or gene. Moreover, with respect to nucleotide sequences, gene-specific oligonucleotide probes comprising 30 or more contiguous nucleotides may be used in sequence-dependent methods of gene identification (e.g., Southern hybridization) and isolation (e.g., in situ hybridization of bacterial colonies or bacteriophage plaques). In addition, short oligonucleotides of 12 or more nucleotides may be used as amplification primers in PCR in order to obtain a particular nucleic acid fragment comprising the primers. Accordingly, a xe2x80x9csubstantial portionxe2x80x9d of a nucleotide sequence comprises a nucleotide sequence that will afford specific identification and/or isolation of a nucleic acid fragment comprising the sequence. The instant specification teaches amino acid and nucleotide sequences encoding polypeptides that comprise one or more particular plant proteins. The skilled artisan, having the benefit of the sequences as reported herein, may now use all or a substantial portion of the disclosed sequences for purposes known to those skilled in this art. Accordingly, the instant invention comprises the complete sequences as reported in the accompanying Sequence Listing, as well as substantial portions of those sequences as defined above.
xe2x80x9cCodon degeneracyxe2x80x9d refers to divergence in the genetic code permitting variation of the nucleotide sequence without effecting the amino acid sequence of an encoded polypeptide. Accordingly, the instant invention relates to any nucleic acid fragment comprising a nucleotide sequence that encodes all or a substantial portion of the amino acid sequences set forth herein. The skilled artisan is well aware of the xe2x80x9ccodon-biasxe2x80x9d exhibited by a specific host cell in usage of nucleotide codons to specify a given amino acid. Therefore, when synthesizing a nucleic acid fragment for improved expression in a host cell, it is desirable to design the nucleic acid fragment such that its frequency of codon usage approaches the frequency of preferred codon usage of the host cell.
xe2x80x9cSynthetic nucleic acid fragmentsxe2x80x9d can be assembled from oligonucleotide building blocks that are chemically synthesized using procedures known to those skilled in the art. These building blocks are ligated and annealed to form larger nucleic acid fragments which may then be enzymatically assembled to construct the entire desired nucleic acid fragment. xe2x80x9cChemically synthesizedxe2x80x9d, as related to nucleic acid fragment, means that the component nucleotides were assembled in vitro. Manual chemical synthesis of nucleic acid fragments may be accomplished using well established procedures, or automated chemical synthesis can be performed using one of a number of commercially available machines. Accordingly, the nucleic acid fragments can be tailored for optimal gene expression based on optimization of nucleotide sequence to reflect the codon bias of the host cell. The skilled artisan appreciates the likelihood of successful gene expression if codon usage is biased towards those codons favored by the host. Determination of preferred codons can be based on a survey of genes derived from the host cell where sequence information is available.
xe2x80x9cGenexe2x80x9d refers to a nucleic acid fragment that expresses a specific protein, including regulatory sequences preceding (5xe2x80x2 non-coding sequences) and following (3xe2x80x2 non-coding sequences) the coding sequence. xe2x80x9cNative genexe2x80x9d refers to a gene as found in nature with its own regulatory sequences. xe2x80x9cChimeric genexe2x80x9d refers any gene that is not a native gene, comprising regulatory and coding sequences that are not found together in nature. Accordingly, a chimeric gene may comprise regulatory sequences and coding sequences that are derived from different sources, or regulatory sequences and coding sequences derived from the same source, but arranged in a manner different than that found in nature. xe2x80x9cEndogenous genexe2x80x9d refers to a native gene in its natural location in the genome of an organism. A xe2x80x9cforeignxe2x80x9d gene refers to a gene not normally found in the host organism, but that is introduced into the host organism by gene transfer. Foreign genes can comprise native genes inserted into a non-native organism, or chimeric genes. A xe2x80x9ctransgenexe2x80x9d is a gene that has been introduced into the genome by a transformation procedure.
xe2x80x9cCoding sequencexe2x80x9d refers to a nucleotide sequence that codes for a specific amino acid sequence. xe2x80x9cRegulatory sequencesxe2x80x9d refer to nucleotide sequences located upstream (5xe2x80x2 non-coding sequences), within, or downstream (3xe2x80x2 non-coding sequences) of a coding sequence, and which influence the transcription, RNA processing or stability, or translation of the associated coding sequence. Regulatory sequences may include promoters, translation leader sequences, introns, and polyadenylation recognition sequences.
xe2x80x9cPromoterxe2x80x9d refers to a nucleotide sequence capable of controlling the expression of a coding sequence or functional RNA. In general, a coding sequence is located 3xe2x80x2 to a promoter sequence. The promoter sequence consists of proximal and more distal upstream elements, the latter elements often referred to as enhancers. Accordingly, an xe2x80x9cenhancerxe2x80x9d is a nucleotide sequence which can stimulate promoter activity and may be an innate element of the promoter or a heterologous element inserted to enhance the level or tissue-specificity of a promoter. Promoters may be derived in their entirety from a native gene, or be composed of different elements derived from different promoters found in nature, or even comprise synthetic nucleotide segments. It is understood by those skilled in the art that different promoters may direct the expression of a gene in different tissues or cell types, or at different stages of development, or in response to different environmental conditions. Promoters which cause a nucleic acid fragment to be expressed in most cell types at most times are commonly referred to as xe2x80x9cconstitutive promotersxe2x80x9d. New promoters of various types useful in plant cells are constantly being discovered; numerous examples may be found in the compilation by Okamuro and Goldberg (1989) Biochemistry of Plants 15:1-82. It is further recognized that since in most cases the exact boundaries of regulatory sequences have not been completely defined, nucleic acid fragments of different lengths may have identical promoter activity.
The xe2x80x9ctranslation leader sequencexe2x80x9d refers to a nucleotide sequence located between the promoter sequence of a gene and the coding sequence. The translation leader sequence is present in the fully processed mRNA upstream of the translation start sequence. The translation leader sequence may affect processing of the primary transcript to mRNA, mRNA stability or translation efficiency. Examples of translation leader sequences have been described (Turner and Foster (1995) Molecular Biotechnology 3:225).
The xe2x80x9c3xe2x80x2 non-coding sequencesxe2x80x9d refer to nucleotide sequences located downstream of a coding sequence and include polyadenylation recognition sequences and other sequences encoding regulatory signals capable of affecting mRNA processing or gene expression. The polyadenylation signal is usually characterized by affecting the addition of polyadenylic acid tracts to the 3xe2x80x2 end of the mRNA precursor. The use of different 3xe2x80x2 non-coding sequences is exemplified by Ingelbrecht et al. (1989) Plant Cell 1:671-680.
xe2x80x9cRNA transcriptxe2x80x9d refers to the product resulting from RNA polymerase-catalyzed transcription of a DNA sequence. When the RNA transcript is a perfect complementary copy of the DNA sequence, it is referred to as the primary transcript or it may be a RNA sequence derived from posttranscriptional processing of the primary transcript and is referred to as the mature RNA. xe2x80x9cMessenger RNA (mRNA)xe2x80x9d refers to the RNA that is without introns and that can be translated into polypeptide by the cell. xe2x80x9ccDNAxe2x80x9d refers to a double-stranded DNA that is complementary to and derived from mRNA. xe2x80x9cSensexe2x80x9d RNA refers to an RNA transcript that includes the mRNA and so can be translated into a polypeptide by the cell. xe2x80x9cAntisense RNAxe2x80x9d refers to an RNA transcript that is complementary to all or part of a target primary transcript or mRNA and that blocks the expression of a target gene (see U.S. Pat. No. 5,107,065, incorporated herein by reference). The complementarity of an antisense RNA may be with any part of the specific nucleotide sequence, i.e., at the 5xe2x80x2 non-coding sequence, 3xe2x80x2 non-coding sequence, introns, or the coding sequence. xe2x80x9cFunctional RNAxe2x80x9d refers to sense RNA, antisense RNA, ribozyme RNA, or other RNA that may not be translated but yet has an effect on cellular processes.
The term xe2x80x9coperably linkedxe2x80x9d refers to the association of two or more nucleic acid fragments on a single nucleic acid fragment so that the function of one is affected by the other. For example, a promoter is operably linked with a coding sequence when it is capable of affecting the expression of that coding sequence (i.e., that the coding sequence is under the transcriptional control of the promoter). Coding sequences can be operably linked to regulatory sequences in sense or antisense orientation.
The term xe2x80x9cexpressionxe2x80x9d, as used herein, refers to the transcription and stable accumulation of sense (mRNA) or antisense RNA derived from the nucleic acid fragment of the invention. Expression may also refer to translation of mRNA into a polypeptide. xe2x80x9cAntisense inhibitionxe2x80x9d refers to the production of antisense RNA transcripts capable of suppressing the expression of the target protein. xe2x80x9cOverexpressionxe2x80x9d refers to the production of a gene product in transgenic organisms that exceeds levels of production in normal or non-transformed organisms. xe2x80x9cCo-suppressionxe2x80x9d refers to the production of sense RNA transcripts capable of suppressing the expression of identical or substantially similar foreign or endogenous genes (U.S. Pat. No. 5,231,020, incorporated herein by reference).
xe2x80x9cAltered levelsxe2x80x9d refers to the production of gene product(s) in transgenic organisms in amounts or proportions that differ from that of normal or non-transformed organisms.
xe2x80x9cMaturexe2x80x9d protein refers to a post-translationally processed polypeptide; i.e., one from which any pre- or propeptides present in the primary translation product have been removed. xe2x80x9cPrecursorxe2x80x9d protein refers to the primary product of translation of mRNA; i.e., with pre- and propeptides still present. Pre- and propeptides may be but are not limited to intracellular localization signals.
A xe2x80x9cchloroplast transit peptidexe2x80x9d is an amino acid sequence which is translated in conjunction with a protein and directs the protein to the chloroplast or other plastid types present in the cell in which the protein is made. xe2x80x9cChloroplast transit sequencexe2x80x9d refers to a nucleotide sequence that encodes a chloroplast transit peptide. A xe2x80x9csignal peptidexe2x80x9d is an amino acid sequence which is translated in conjunction with a protein and directs the protein to the secretory system (Chrispeels (1991) Ann. Rev. Plant Phys. Plant Mol. Biol. 42:21-53). If the protein is to be directed to a vacuole, a vacuolar targeting signal (supra) can further be added, or if to the endoplasmic reticulum, an endoplasmic reticulum retention signal (supra) may be added. If the protein is to be directed to the nucleus, any signal peptide present should be removed and instead a nuclear localization signal included (Raikhel (1992) Plant Phys. 100:1627-1632).
xe2x80x9cTransformationxe2x80x9d refers to the transfer of a nucleic acid fragment into the genome of a host organism, resulting in genetically stable inheritance. Host organisms containing the transformed nucleic acid fragments are referred to as xe2x80x9ctransgenicxe2x80x9d organisms. Examples of methods of plant transformation include Agrobacterium-mediated transformation (De Blaere et al. (1987) Meth. Enzymol. 143:277) and particle-accelerated or xe2x80x9cgene gunxe2x80x9d transformation technology (Klein et al. (1987) Nature (London) 327:70-73; U.S. Pat. No. 4,945,050, incorporated herein by reference).
Standard recombinant DNA and molecular cloning techniques used herein are well known in the art and are described more fully in Sambrook et al. Molecular Cloning: A Laboratory Manual; Cold Spring Harbor Laboratory Press: Cold Spring Harbor, 1989 (hereinafter xe2x80x9cManiatisxe2x80x9d).
Nucleic acid fragments encoding at least a portion of several branched-chain amino acid degradation enzymes have been isolated and identified by comparison of random plant cDNA sequences to public databases containing nucleotide and protein sequences using the BLAST algorithms well known to those skilled in the art. The nucleic acid fragments of the instant invention may be used to isolate cDNAs and genes encoding homologous proteins from the same or other plant species. Isolation of homologous genes using sequence-dependent protocols is well known in the art. Examples of sequence-dependent protocols include, but are not limited to, methods of nucleic acid hybridization, and methods of DNA and RNA amplification as exemplified by various uses of nucleic acid amplification technologies (e.g., polymerase chain reaction, ligase chain reaction).
For example, genes encoding other hydroxymethylglutaryl CoA oxidases, 3-hydroxyisobutyrate dehydrogenases or isovalyryl-CoA dehydrogenases, either as cDNAs or genomic DNAs, could be isolated directly by using all or a portion of the instant nucleic acid fragments as DNA hybridization probes to screen libraries from any desired plant employing methodology well known to those skilled in the art. Specific oligonucleotide probes based upon the instant nucleic acid sequences can be designed and synthesized by methods known in the art (Maniatis). Moreover, the entire sequences can be used directly to synthesize DNA probes by methods known to the skilled artisan such as random primer DNA labeling, nick translation, or end-labeling techniques, or RNA probes using available in vitro transcription systems. In addition, specific primers can be designed and used to amplify a part or all of the instant sequences. The resulting amplification products can be labeled directly during amplification reactions or labeled after amplification reactions, and used as probes to isolate full length cDNA or genomic fragments under conditions of appropriate stringency.
In addition, two short segments of the instant nucleic acid fragments may be used in polymerase chain reaction protocols to amplify longer nucleic acid fragments encoding homologous genes from DNA or RNA. The polymerase chain reaction may also be performed on a library of cloned nucleic acid fragments wherein the sequence of one primer is derived from the instant nucleic acid fragments, and the sequence of the other primer takes advantage of the presence of the polyadenylic acid tracts to the 3xe2x80x2 end of the mRNA precursor encoding plant genes. Alternatively, the second primer sequence may be based upon sequences derived from the cloning vector. For example, the skilled artisan can follow the RACE protocol (Frohman et al. (1988) Proc. Natl. Acad. Sci. USA 85:8998) to generate cDNAs by using PCR to amplify copies of the region between a single point in the transcript and the 3xe2x80x2 or 5xe2x80x2 end. Primers oriented in the 3xe2x80x2 and 5xe2x80x2 directions can be designed from the instant sequences. Using commercially available 3xe2x80x2 RACE or 5xe2x80x2 RACE systems (BRL), specific 3xe2x80x2 or 5xe2x80x2 cDNA fragments can be isolated (Ohara et al. (1989) Proc. Natl. Acad. Sci. USA 86:5673; Loh et al. (1989) Science 243:217). Products generated by the 3xe2x80x2 and 5xe2x80x2 RACE procedures can be combined to generate full-length cDNAs (Frohman and Martin (1989) Techniques 1: 165).
Availability of the instant nucleotide and deduced amino acid sequences facilitates immunological screening of cDNA expression libraries. Synthetic peptides representing portions of the instant amino acid sequences may be synthesized. These peptides can be used to immunize animals to produce polyclonal or monoclonal antibodies with specificity for peptides or proteins comprising the amino acid sequences. These antibodies can be then be used to screen cDNA expression libraries to isolate full-length cDNA clones of interest (Lerner (1984) Adv. Immunol. 36:1; Maniatis).
The nucleic acid fragments of the instant invention may be used to create transgenic plants in which the disclosed polypeptides are present at higher or lower levels than normal or in cell types or developmental stages in which they are not normally found. This would have the effect of altering the level of valine, leucine, or isoleucine in those cells. This may result in the accumulation of toxic compounds such as 3-hydroxyisobutyrate which would be a useful heribicide target.
Overexpression of the proteins of the instant invention may be accomplished by first constructing a chimeric gene in which the coding region is operably linked to a promoter capable of directing expression of a gene in the desired tissues at the desired stage of development. For reasons of convenience, the chimeric gene may comprise promoter sequences and translation leader sequences derived from the same genes. 3xe2x80x2 Non-coding sequences encoding transcription termination signals may also be provided. The instant chimeric gene may also comprise one or more introns in order to facilitate gene expression.
Plasmid vectors comprising the instant chimeric gene can then be constructed. The choice of plasmid vector is dependent upon the method that will be used to transform host plants. The skilled artisan is well aware of the genetic elements that must be present on the plasmid vector in order to successfully transform, select and propagate host cells containing the chimeric gene. The skilled artisan will also recognize that different independent transformation events will result in different levels and patterns of expression (Jones et al. (1985) EMBO J. 4:2411-2418; De Almeida et al. (1989) Mol. Gen. Genetics 218:78-86), and thus that multiple events must be screened in order to obtain lines displaying the desired expression level and pattern. Such screening may be accomplished by Southern analysis of DNA, Northern analysis of mRNA expression, Western analysis of protein expression, or phenotypic analysis.
For some applications it may be useful to direct the instant polypeptides to different cellular compartments, or to facilitate its secretion from the cell. It is thus envisioned that the chimeric gene described above may be further supplemented by altering the coding sequence to encode the instant polypeptides with appropriate intracellular targeting sequences such as transit sequences (Keegstra (1989) Cell 56:247-253), signal sequences or sequences encoding endoplasmic reticulum localization (Chrispeels (1991) Ann. Rev. Plant Phys. Plant Mol. Biol 42:21-53), or nuclear localization signals (Raikhel (1992) Plant Phys. 100:1627-1632) added and/or with targeting sequences that are already present removed. While the references cited give examples of each of these, the list is not exhaustive and more targeting signals of utility may be discovered in the future.
It may also be desirable to reduce or eliminate expression of genes encoding the instant polypeptides in plants for some applications. In order to accomplish this, a chimeric gene designed for co-suppression of the instant polypeptide can be constructed by linking a gene or gene fragment encoding that polypeptide to plant promoter sequences. Alternatively, a chimeric gene designed to express antisense RNA for all or part of the instant nucleic acid fragment can be constructed by linking the gene or gene fragment in reverse orientation to plant promoter sequences. Either the co-suppression or antisense chimeric genes could be introduced into plants via transformation wherein expression of the corresponding endogenous genes are reduced or eliminated.
Molecular genetic solutions to the generation of plants with altered gene expression have a decided advantage over more traditional plant breeding approaches. Changes in plant phenotypes can be produced by specifically inhibiting expression of one or more genes by antisense inhibition or cosuppression (U.S. Pat. Nos. 5,190,931, 5,107,065 and 5,283,323). An antisense or cosuppression construct would act as a dominant negative regulator of gene activity. While conventional mutations can yield negative regulation of gene activity these effects are most likely recessive. The dominant negative regulation available with a transgenic approach may be advantageous from a breeding perspective. In addition, the ability to restrict the expression of specific phenotype to the reproductive tissues of the plant by the use of tissue specific promoters may confer agronomic advantages relative to conventional mutations which may have an effect in all tissues in which a mutant gene is ordinarily expressed.
The person skilled in the art will know that special considerations are associated with the use of antisense or cosuppresion technologies in order to reduce expression of particular genes. For example, the proper level of expression of sense or antisense genes may require the use of different chimeric genes utilizing different regulatory elements known to the skilled artisan. Once transgenic plants are obtained by one of the methods described above, it will be necessary to screen individual transgenics for those that most effectively display the desired phenotype. Accordingly, the skilled artisan will develop methods for screening large numbers of transformants. The nature of these screens will generally be chosen on practical grounds, and is not an inherent part of the invention. For example, one can screen by looking for changes in gene expression by using antibodies specific for the protein encoded by the gene being suppressed, or one could establish assays that specifically measure enzyme activity. A preferred method will be one which allows large numbers of samples to be processed rapidly, since it will be expected that a large number of transformants will be negative for the desired phenotype.
The instant polypeptides (or portions thereof) may be produced in heterologous host cells, particularly in the cells of microbial hosts, and can be used to prepare antibodies to the these proteins by methods well known to those skilled in the art. The antibodies are useful for detecting the polypeptides of the instant invention in situ in cells or in vitro in cell extracts. Preferred heterologous host cells for production of the instant polypeptides are microbial hosts. Microbial expression systems and expression vectors containing regulatory sequences that direct high level expression of foreign proteins are well known to those skilled in the art. Any of these could be used to construct a chimeric gene for production of the instant polypeptides. This chimeric gene could then be introduced into appropriate microorganisms via transformation to provide high level expression of the encoded branched-chain amino acid degradation enzymes. An example of a vector for high level expression of the instant polypeptides in a bacterial host is provided (Example 8).
Additionally, the instant polypeptides can be used as targets to facilitate design and/or identification of inhibitors of those enzymes that may be useful as herbicides. This is desirable because the polypeptides described herein catalyze various steps in degradation of branched-chain amino acids. Accordingly, inhibition of the activity of one or more of the enzymes described herein could lead to inhibition plant growth. Thus, the instant polypeptides could be appropriate for new herbicide discovery and design.
All or a substantial portion of the nucleic acid fragments of the instant invention may also be used as probes for genetically and physically mapping the genes that they are a part of, and as markers for traits linked to those genes. Such information may be useful in plant breeding in order to develop lines with desired phenotypes. For example, the instant nucleic acid fragments may be used as restriction fragment length polymorphism (RFLP) markers. Southern blots (Maniatis) of restriction-digested plant genomic DNA may be probed with the nucleic acid fragments of the instant invention. The resulting banding patterns may then be subjected to genetic analyses using computer programs such as MapMaker (Lander et al. (1987) Genomics 1:174-181) in order to construct a genetic map. In addition, the nucleic acid fragments of the instant invention may be used to probe Southern blots containing restriction endonuclease-treated genomic DNAs of a set of individuals representing parent and progeny of a defined genetic cross. Segregation of the DNA polymorphisms is noted and used to calculate the position of the instant nucleic acid sequence in the genetic map previously obtained using this population (Botstein et al. (1980) Am. J. Hum. Genet. 32:314-331).
The production and use of plant gene-derived probes for use in genetic mapping is described in Bernatzky and Tanksley (1986) Plant Mol. Biol. Reporter 4(1):37-41. Numerous publications describe genetic mapping of specific cDNA clones using the methodology outlined above or variations thereof. For example, F2 intercross populations, backcross populations, randomly mated populations, near isogenic lines, and other sets of individuals may be used for mapping. Such methodologies are well known to those skilled in the art.
Nucleic acid probes derived from the instant nucleic acid sequences may also be used for physical mapping (i.e., placement of sequences on physical maps; see Hoheisel et al. In: Nonmammalian Genomic Analysis: A Practical Guide, Academic press 1996, pp. 319-346, and references cited therein).
In another embodiment, nucleic acid probes derived from the instant nucleic acid sequences may be used in direct fluorescence in situ hybridization (FISH) mapping (Trask (1991) Trends Genet. 7:149-154). Although current methods of FISH mapping favor use of large clones (several to several hundred KB; see Laan et al. (1995) Genome Research 5:13-20), improvements in sensitivity may allow performance of FISH mapping using shorter probes.
A variety of nucleic acid amplification-based methods of genetic and physical mapping may be carried out using the instant nucleic acid sequences. Examples include allele-specific amplification (Kazazian (1989) J. Lab. Clin. Med. 114(2):95-96), polymorphism of PCR-amplified fragments (CAPS; Sheffield et al. (1993) Genomics 16:325-332), allele-specific ligation (Landegren et al. (1988) Science 241:1077-1080), nucleotide extension reactions (Sokolov (1990) Nucleic Acid Res. 18:3671), Radiation Hybrid Mapping (Walter et al. (1997) Nature Genetics 7:22-28) and Happy Mapping (Dear and Cook (1989) Nucleic Acid Res. 17:6795-6807). For these methods, the sequence of a nucleic acid fragment is used to design and produce primer pairs for use in the amplification reaction or in primer extension reactions. The design of such primers is well known to those skilled in the art. In methods employing PCR-based genetic mapping, it may be necessary to identify DNA sequence differences between the parents of the mapping cross in the region corresponding to the instant nucleic acid sequence. This, however, is generally not necessary for mapping methods.
Loss of function mutant phenotypes may be identified for the instant cDNA clones either by targeted gene disruption protocols or by identifying specific mutants for these genes contained in a maize population carrying mutations in all possible genes (Ballinger and Benzer (1989) Proc. Natl. Acad. Sci USA 86:9402; Koes et al. (1995) Proc. Natl. Acad. Sci USA 92:8149; Bensen et al. (1995) Plant Cell 7:75). The latter approach may be accomplished in two ways. First, short segments of the instant nucleic acid fragments may be used in polymerase chain reaction protocols in conjunction with a mutation tag sequence primer on DNAs prepared from a population of plants in which Mutator transposons or some other mutation-causing DNA element has been introduced (see Bensen, supra). The amplification of a specific DNA fragment with these primers indicates the insertion of the mutation tag element in or near the plant gene encoding the instant polypeptides. Alternatively, the instant nucleic acid fragment may be used as a hybridization probe against PCR amplification products generated from the mutation population using the mutation tag sequence primer in conjunction with an arbitrary genomic site primer, such as that for a restriction enzyme site-anchored synthetic adaptor. With either method, a plant containing a mutation in the endogenous gene encoding the instant polypeptides can be identified and obtained. This mutant plant can then be used to determine or confirm the natural function of the instant polypeptides disclosed herein.